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            <div><h3 class="title">
                <a name="asio.tutorial.tuttimer5"></a><a class="link" href="tuttimer5.html"
                                                         title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5
                - Synchronising handlers
                in multithreaded programs</a>
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    <p>
        This tutorial demonstrates the use of the <a class="link" href="../reference/strand.html"
                                                     title="strand">strand</a>
        class template to synchronise callback handlers in a multithreaded program.
    </p>
    <p>
        The previous four tutorials avoided the issue of handler synchronisation
        by calling the <a class="link" href="../reference/io_context/run.html"
                          title="io_context::run">io_context::run()</a>
        function from one thread only. As you already know, the asio library provides
        a guarantee that callback handlers will only be called from threads that
        are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        Consequently, calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        from only one thread ensures that callback handlers cannot run concurrently.
    </p>
    <p>
        The single threaded approach is usually the best place to start when developing
        applications using asio. The downside is the limitations it places on programs,
        particularly servers, including:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                Poor responsiveness when handlers can take a long time to complete.
            </li>
            <li class="listitem">
                An inability to scale on multiprocessor systems.
            </li>
        </ul>
    </div>
    <p>
        If you find yourself running into these limitations, an alternative approach
        is to have a pool of threads calling <a class="link" href="../reference/io_context/run.html"
                                                title="io_context::run">io_context::run()</a>.
        However, as this allows handlers to execute concurrently, we need a method
        of synchronisation when handlers might be accessing a shared, thread-unsafe
        resource.
    </p>
    <pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span
            class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span
                class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span
                class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span
                class="identifier">boost</span><span class="special">/</span><span class="identifier">bind</span><span
                class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span
                class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
    <p>
        We start by defining a class called <code class="computeroutput"><span class="identifier">printer</span></code>,
        similar to the class in the previous tutorial. This class will extend the
        previous tutorial by running two timers in parallel.
    </p>
    <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">printer</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
    <p>
        In addition to initialising a pair of asio::steady_timer members, the constructor
        initialises the <code class="computeroutput"><span class="identifier">strand_</span></code> member,
        an object of type asio::strand&lt;asio::io_context::executor_type&gt;.
    </p>
    <p>
        The <a class="link" href="../reference/strand.html" title="strand">strand</a> class template is
        an executor adapter that guarantees that, for those handlers that are dispatched
        through it, an executing handler will be allowed to complete before the next
        one is started. This is guaranteed irrespective of the number of threads
        that are calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        Of course, the handlers may still execute concurrently with other handlers
        that were not dispatched through an <a class="link" href="../reference/strand.html" title="strand">strand</a>,
        or were dispatched through a different <a class="link" href="../reference/strand.html" title="strand">strand</a>
        object.
    </p>
    <pre class="programlisting">  <span class="identifier">printer</span><span class="special">(</span><span
            class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span
            class="special">&amp;</span> <span class="identifier">io</span><span class="special">)</span>
    <span class="special">:</span> <span class="identifier">strand_</span><span class="special">(</span><span
                class="identifier">asio</span><span class="special">::</span><span class="identifier">make_strand</span><span
                class="special">(</span><span class="identifier">io</span><span class="special">)),</span>
      <span class="identifier">timer1_</span><span class="special">(</span><span class="identifier">io</span><span
                class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">chrono</span><span class="special">::</span><span
                class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span
                class="special">)),</span>
      <span class="identifier">timer2_</span><span class="special">(</span><span class="identifier">io</span><span
                class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">chrono</span><span class="special">::</span><span
                class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span
                class="special">)),</span>
      <span class="identifier">count_</span><span class="special">(</span><span class="number">0</span><span
                class="special">)</span>
  <span class="special">{</span>
</pre>
    <p>
        When initiating the asynchronous operations, each callback handler is "bound"
        to an asio::strand&lt;asio::io_context::executor_type&gt; object. The asio::bind_executor()
        function returns a new handler that automatically dispatches its contained
        handler through the <a class="link" href="../reference/strand.html" title="strand">strand</a> object.
        By binding the handlers to the same <a class="link" href="../reference/strand.html" title="strand">strand</a>,
        we are ensuring that they cannot execute concurrently.
    </p>
    <pre class="programlisting">    <span class="identifier">timer1_</span><span class="special">.</span><span
            class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span
            class="special">::</span><span class="identifier">bind_executor</span><span class="special">(</span><span
            class="identifier">strand_</span><span class="special">,</span>
          <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span
                class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span
                class="identifier">print1</span><span class="special">,</span> <span class="keyword">this</span><span
                class="special">)));</span>

    <span class="identifier">timer2_</span><span class="special">.</span><span class="identifier">async_wait</span><span
                class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">bind_executor</span><span class="special">(</span><span
                class="identifier">strand_</span><span class="special">,</span>
          <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span
                class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span
                class="identifier">print2</span><span class="special">,</span> <span class="keyword">this</span><span
                class="special">)));</span>
  <span class="special">}</span>

  <span class="special">~</span><span class="identifier">printer</span><span class="special">()</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Final count is "</span> <span class="special">&lt;&lt;</span> <span
                class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span
                class="special">;</span>
  <span class="special">}</span>
</pre>
    <p>
        In a multithreaded program, the handlers for asynchronous operations should
        be synchronised if they access shared resources. In this tutorial, the shared
        resources used by the handlers (<code class="computeroutput"><span class="identifier">print1</span></code>
        and <code class="computeroutput"><span class="identifier">print2</span></code>) are <code
            class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span
            class="identifier">cout</span></code>
        and the <code class="computeroutput"><span class="identifier">count_</span></code> data member.
    </p>
    <pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">print1</span><span
            class="special">()</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span
                class="special">&lt;</span> <span class="number">10</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Timer 1: "</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
      <span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>

      <span class="identifier">timer1_</span><span class="special">.</span><span
                class="identifier">expires_at</span><span class="special">(</span><span
                class="identifier">timer1_</span><span class="special">.</span><span
                class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span
                class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span
                class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span
                class="number">1</span><span class="special">));</span>

      <span class="identifier">timer1_</span><span class="special">.</span><span
                class="identifier">async_wait</span><span class="special">(</span><span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">bind_executor</span><span class="special">(</span><span
                class="identifier">strand_</span><span class="special">,</span>
            <span class="identifier">boost</span><span class="special">::</span><span
                class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span
                class="special">::</span><span class="identifier">print1</span><span class="special">,</span> <span
                class="keyword">this</span><span class="special">)));</span>
    <span class="special">}</span>
  <span class="special">}</span>

  <span class="keyword">void</span> <span class="identifier">print2</span><span class="special">()</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span
                class="special">&lt;</span> <span class="number">10</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Timer 2: "</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
      <span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>

      <span class="identifier">timer2_</span><span class="special">.</span><span
                class="identifier">expires_at</span><span class="special">(</span><span
                class="identifier">timer2_</span><span class="special">.</span><span
                class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span
                class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span
                class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span
                class="number">1</span><span class="special">));</span>

      <span class="identifier">timer2_</span><span class="special">.</span><span
                class="identifier">async_wait</span><span class="special">(</span><span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">bind_executor</span><span class="special">(</span><span
                class="identifier">strand_</span><span class="special">,</span>
            <span class="identifier">boost</span><span class="special">::</span><span
                class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span
                class="special">::</span><span class="identifier">print2</span><span class="special">,</span> <span
                class="keyword">this</span><span class="special">)));</span>
    <span class="special">}</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">strand</span><span
                class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">io_context</span><span class="special">::</span><span class="identifier">executor_type</span><span
                class="special">&gt;</span> <span class="identifier">strand_</span><span class="special">;</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span
                class="identifier">timer1_</span><span class="special">;</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span
                class="identifier">timer2_</span><span class="special">;</span>
  <span class="keyword">int</span> <span class="identifier">count_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
    <p>
        The <code class="computeroutput"><span class="identifier">main</span></code> function now causes
        <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> to
        be called from two threads: the main thread and one additional thread. This
        is accomplished using an <a class="link" href="../reference/thread.html" title="thread">thread</a>
        object.
    </p>
    <p>
        Just as it would with a call from a single thread, concurrent calls to <a class="link"
                                                                                  href="../reference/io_context/run.html"
                                                                                  title="io_context::run">io_context::run()</a>
        will continue
        to execute while there is "work" left to do. The background thread
        will not exit until all asynchronous operations have completed.
    </p>
    <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span
            class="special">()</span>
<span class="special">{</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span
                class="identifier">io</span><span class="special">;</span>
  <span class="identifier">printer</span> <span class="identifier">p</span><span class="special">(</span><span
                class="identifier">io</span><span class="special">);</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">thread</span> <span
                class="identifier">t</span><span class="special">(</span><span class="identifier">boost</span><span
                class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">io_context</span><span class="special">::</span><span
                class="identifier">run</span><span class="special">,</span> <span class="special">&amp;</span><span
                class="identifier">io</span><span class="special">));</span>
  <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span
                class="special">();</span>
  <span class="identifier">t</span><span class="special">.</span><span class="identifier">join</span><span
                class="special">();</span>

  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
    <p>
        See the <a class="link" href="tuttimer5/src.html" title="Source listing for Timer.5">full source listing</a>
    </p>
    <p>
        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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